Time for acquiring a new gene by duplication.

In view of the widespread occurrence of gene families in eukaryotic genomes that suggests the importance of gene duplication in evolution, a population genetic model incorporating unequal crossing-over was formulated. By using this model, the time needed for acquiring a new gene is investigated by an approximate analytical method and by computer simulations. The model assumes that natural selection favors those chromosomes with more beneficial genes than other chromosomes in the population, as well as random genetic drift, mutation, and unequal crossing-over. Starting from a single gene copy, it is found that the time for acquiring another gene with a new function is dependent on the rates of occurrence of unequal crossing-over and mutation. Within a realistic range of parameter values, the required time was at least several times 4N generations, where N is the effective population size. Interchromosomal unequal crossing-over at meiosis is more effective than intrachromosomal (between sister chromatids) unequal crossing-over for obtaining a new gene, provided that other parameters are the same. However, the genetic load for acquiring a gene is larger under the model of interchromosomal crossing-over. The relevance of this finding to the advantage of sexual reproduction is discussed.